On all steel products shipped from the workshop for customers I will regularly use Swarfega Duck Oil as the last coating before being packaged and sent out.
Duck Oil is essentially Swarfega’s brand of 3-1, which works just as well. I switched to duck oil when I found out that WD40 was in fact hygroscopic. Hygroscopic substances can have the ability to attract and hold onto water from the surrounding atmosphere or environments – for armour and steel this is not a good idea. WD40 can be used to help remove light rusting but should always be cleaned off afterwards and proper oil applied to the again bare surface. Should it be left on for a protracted period it can in fact accelerate rusting and problems with dirt. WD40 can also cause damage to leather work.
I have found over the years that Duck Oil has a nice viscosity to it that lends itself well to holding onto a steel surface and acting as a barrier to rust. Metal cleaning products such as autosol, brasso, silvo &c. remove it from the surface material with a little bit of elbow grease. I have not noticed it cause any damage to leather.
The only downside of Duck Oil I have found to date is that it can give the illusion of a matt steel finish to mirrored or bright finish. This is easily regained with the cleaning products mentioned above, but will again leave the surface of the armour bare to the elements. Using a light coat of the duck oil on a rag can help to keep out moisture and retain 90% of the mirrored finish.
All in all a very versatile product that protects, cleans a lubricates metal surfaces well.